Much effort and resources are expended in ongoing corrosion control, even when an item has been coated initially to protect it. Abrasion, saltwater, high humidity, ultraviolet light, temperature gradients, and other deleterious environmental factors contribute to this, alone or in combination. To conserve corrosion control resources, it would be beneficial to use not only a “tough” initial protective coating, but also a coating that, when compromised, repairs or “heals” itself.
There have been efforts to effect a self-repairing capability in various materials, notably shaped articles that may be made of materials with a weakness in one or more orientations, such as cementitious materials having inherently poor tensile strength. U.S. Pat. No. 5,575,841, Cementitious Materials, to Dry, Nov. 19, 1996, and U.S. Pat. No. 5,660,624 (Aug. 26, 1997), U.S. Pat. No. 5,989,334 (Nov. 23, 1999), and U.S. Pat. No. 6,261,360 B1 (Jul. 17, 2001), each entitled Self-Repairing Reinforced Matrix Materials, all to Dry, detail a method of incorporating hollow fibers in “pourable” material to effect a self-repairing function. These inventions employ selectively releasable compounds within the hollow fiber. Because of the size of the fibers, these patents are unsuitable for repair upon a smooth surface.
Microcapsules contain minute amounts of product for specialized delivery, often size, time or location critical. They may be obtained in diameters of less than 250 microns (μ) and have been used in a variety of applications, from the pharmaceutical industry (delivery of drugs) to the textile industry (providing protective wear for HAZMAT workers). One example is U.S. Pat. No. 6,060,152, Fabric with Microencapsulated Breach Indication Coating, to Murchie, May 9, 2000. The '152 patent describes a membrane incorporating a number of different microcapsules that alert to even the smallest compromise of the fabric comprising a protective suit such as may be worn by a HAZMAT worker or health professional.
Very recent work to improve coatings by the addition of additives involves only improving the application of the coating to a substrate, not the ability of the coating to “repair” itself upon its compromise. One such example is U.S. patent application no. 2002/0106454 A1, High Molecular Weight Polymer Additive for Coating and Protective Products, to Trippe et al., Published Aug. 8, 2002. The '454 published application details the advantages of adding small amounts of an ultrahigh molecular weight polymer, such as polyisobutylene, to enhance coating properties of a solvent. Once, a nick compromises the coating or abrasion, however, another separately applied application is required to protect the substrate.
Another concern in using microcapsules with solvents is the timing of delivery of the encapsulated compound. Prior patents have avoided this timing problem by mixing the microcapsules with dry powder coatings such as are used in “powder coating” applications involving elevated temperatures. U.S. Pat. No. 6,075,072, Latent Coating for Metal Surface Repair, to Guilbert et al., Jun. 13, 2000, details a self-repairing compound suitable for use in powder coating. By adding microcapsules of sufficiently small size, i.e., 10–40μ, to a dry powder form of protective coating, the resultant “self-repairing” coating is able to be powder coated upon metal substrates at suitable elevated temperatures that melt the coating to a homogenous continuous surface of approximately 200μ thickness. The microcapsules used with the '072 patent are mixed with a solvent in the dry state to prevent short-term degradation of the shells by liquid solvents.
Unless appropriate materials are used to fabricate the microcapsule and its contents, it may “deploy” before the coating is applied or, upon application, spontaneously deploy improperly, i.e., without a physical compromise of the coating such as abrasion or nicking. Further, unless the microcapsule is compatible with both its contents (the encapsulated repair compound) and its surrounds (the solvent), the “application” life of the resultant mixed product may be less than desirable. These constraints have been addressed in the present invention.